Various forms of planar electrical interconnects are used for computer and electrical systems. Typically these interconnects are characterized by large numbers of contact points with the contact resistance dependant on several factors including the applied normal force on the interconnect. Good electrical contact across a planar interconnect is critical to realizing high reliability from these designs and ensuring satisfactory overall electronic system performance. A measurement of the applied normal force distribution across an interconnect provides an excellent means for the determination of the quality and reliability of the contact at the interconnect surface. Optical techniques may be used for measuring these forces and offers the ability to easily characterize large high density interconnects.
U.S. Pat. No. 5,177,555 issued to Stratton et al. discloses a method and apparatus for optically determining a qualitative distribution of the normal force present at mating surfaces of an electrical surface connector. The Stratton patent uses photoelastic members that have plane-polarized, narrow wavelength light coupled into each member through optical fibers. The temporarily birefringent photoelastic material provides a relative propagation delay between vector components of the light in each member. The light exiting each member is input to a plane polarizer, and then fed through a series of fibers to an intensity meter. The relative delay between the vector components creates a detectable change in intensity at the intensity meter. The Stratton method produces a sinusoidal plot of intensity variation with pressure but due to fringe ambiguity does not tell exactly how much the exerted pressure is. The force distribution data obtained by using the Stratton technique is purely qualitative.